1. Field of the Present Invention
The present invention relates to methods and inspection apparatus for determining an overlay error, for example in the manufacture of devices by lithographic techniques using a lithographic apparatus.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between two layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-destructive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Semiconductor device manufacturers align wafers using gratings that are present on a wafer. An alignment sensor measures the location of a grating with sub-nm repeatability. The manufacturers also measure on-product overlay using overlapping gratings. Here sub-nm Total Measurement Uncertainty (TMU) numbers are easily achieved as well. However, overlay metrology and alignment sensors are sensitive to marker asymmetry caused by processing steps like etch, Chemical Mechanical Polishing (CMP) and deposition. These asymmetries lead to overlay and alignment errors that are of the order of a few nm's. This effect starts to dominate the overlay budget and solutions are therefore needed.
Scatterometer measurement recipe selection (for example with each recipe having various wavelengths and polarizations of illumination) is currently performed using parameters such as mean Tool Induced Shift (TIS) and or TIS variability (a.k.a. TIS 3 sigma). There is a problem when the reference layer exhibits an asymmetrical profile.
Asymmetries in the shape of a target grating will generally have an impact on the measured overlay. This impact can vary depending on the illumination setting used for the measurement.
Target recipe selection is performed without actual knowledge of the shape of the gratings after processing and imaging. Furthermore, the context of the current process is not used in the decision of recipe selection. The use of qualifiers that are based on TIS and/or TMU do not always lead to a measurement recipe that is most robust against target asymmetry.